In a wireless communication system, in order for a user device to transmit and receive data over the air interface, the user device needs to be allocated physical radio resources, and the user device must be notified of its resource assignments by means of control signalling. In the Long Term Evolution (LTE) of UTRAN (UMTS (Universal Mobile Telecommunications System) Terrestrial Radio Access Network), the physical downlink control channel (PDCCH) carries scheduling assignments and other control information, and the PDCCH resources used to signal scheduling assignments to users in the cell are dynamically allocated by the serving base station.
A PDCCH consists of an aggregation of one or more consecutive control channel elements (CCEs), where a CCE occupies a fraction of the base station cell's available physical radio resources. The total number of CCEs available in a cell depends on the system bandwidth of the cell and the number of OFDM (Orthogonal Frequency Division Multiplexing) symbols reserved for PDCCH transmission in a given sub-frame. A given CCE can be used to signal either an UL (Uplink) or DL (Downlink) resource allocation, and the CCE resources in a cell must be shared between DL and UL schedulers. Candidate user devices that fail to receive a CCE allocation are said to be blocked. CCE blocking has an adverse effect on the overall cell throughput, because it means that the main scheduler is operating on a reduced and sub-optimal list of candidate user devices.
In one known method of CCE allocation, it is assumed that the UL and DL CCE assignments are performed concurrently. CCE resources are allocated to UL and DL users in turn. This results in a more or less equal probability of “blocking” between the UL and the DL. However, in some implementations, the UL and DL schedulers run on physically separate processors and execute at different times within the sub-frame. In this the CCE allocation processing for UL and DL cannot be performed concurrently.
In another known method of CCE allocation, DL users (i.e. user devices that have downlink data to be transmitted to them) are allocated first until the number of CCEs allocated for the DL is greater than or equal to a certain threshold, and then UL users (i.e. user devices that have uplink data to transmit) are assigned CCEs among the CCEs left unused by the DL. The advantage of this method is that it does not require UL and DL CCE assignments to be performed concurrently, and so the UL and DL schedulers can execute at different times. However this method has the disadvantage of a higher probability of UL CCE blocking, since the degrees of freedom for CCE UL allocation are restricted by the CCE allocation already made for the DL.
There is therefore a need for alternative methods and apparatus for CCE allocation.